For preventing environmental pollution by motor vehicles there has been devised techniques preventing escape of evaporated fuel gases and vapors in addition to techniques for treating exhaust gases. For example, it is known to dispose a canister containing activated charcoal between a float chamber of a carburetor and a vapor separator of a fuel tank so that evaporated fuel gases and vapors in the fuel tank and in the float chamber are absorbed in the activated carbon in the canister, while the ignition key of the engine is OFF. While the engine is in the running state, the evaporated fuel gases in the canister are led into an engine by the negative pressure of an intake manifold.
More specifically, as conventional apparatus coping with the escape of fuel gases and vapors in automobiles, there has been provided an electromagnetic valve operating mechanism disposed between a charcoal canister and the float chamber of a carburetor. The valve is opened whenever the ignition key of the engine is OFF.
However in conventional known evaporative emission systems of the above-mentioned type, if the ignition key is kept in the OFF state continuously for a long period, such as 2 or 3 days, the low-boiling-point components (easily gasifiable components) in the gasoline of the float chamber 3 are reduced, and the starting characteristics of the engine are degraded, a distinct disadvantage.